What I found:
Select
should beFALSE
when comparing with AICSelect=TRUE
ONLY when we have a single model in mind (instead of comparing many models with AIC)All models being compared should be fit with
method=ML
when unpenalized fixed effects (parametric terms) are present (like here)Method=REML
is fine though for comparing "nested" GAM(M) models in AIC when all effects are fully penalized
Found a great tutorial on GAMs from Gavin here which covers most of this.